Even in the best case where Debian satisfies all the reproducibility goals it sets for itself (i.e. the ~96% at https://tests.reproducible-builds.org/debian/reproducible.ht... becomes 100%), each Debian package is still only a reproducible function of the current versions of its build dependencies at the time it was built. Upgrading those dependencies can change the build output, and this is still normal and expected in Debian; it’s the entire point of a binNMU.

In order to reproduce a given Debian package from scratch, one would need to track down long chains of historical versions of packages from sid. Due to circular dependencies, these chains probably snake back to the very first versions of Debian before the availability of snapshot.debian.org. It’s possible that human judgment could be used to break some chains at an earlier point by substituting different versions, but this goes outside the guarantees provided by Debian’s reproducibility.

Nix guarantees that every package is built with the specified versions of its dependencies, and so on recursively. Some more rebuilding is necessary to maintain this guarantee, but in exchange, it enables stronger notions of reproducibility that Debian can’t.